Skin is the largest organ in the human body. Weighing roughly nine pounds on the average adult, it protects the body from harmful pollutants found in air, water and other things people come in contact with every day. Skin performs many other functions, too. It helps regulate body temperature, houses sensory receptors that help you feel things and synthesizes various body chemicals necessary for life. That’s why the condition of the skin is so important to good health.

Skin has many sections, but it basically is divided into three layers:

The top layer, or epidermis, is the one that produces a tan.
The middle layer, or dermis, contains collagen and other elastic materials important to the skin’s strength, and to its ability to fight off infection and repair itself. Blood vessels, nerve fibers and other structures are embedded in this layer.
The bottom layer, or subcutaneous tissue, primarily is composed of fat that binds the skin to the body. Subcutaneous tissue serves as the body’s food reserve, insulation and shock absorber.
Skin cells in the epidermis are constantly reproducing and pushing older cells upward to the surface of your skin – an outer mantle of dead skin cells (sometimes called the horny layer) where they are sloughed off in about one month. There are three main types of cells in the epidermis:

Basal cells — the oblong cells that line the base of the germinative layer — are parent cells, giving “birth” to keratinocytes.

Keratinocytes are the “daughter” cells that serve as your skin’s sealant, making up most of your epidermis.

About 5 percent of the skin cells in the epidermis are special cells called melanocytes, which lie on the bottom of the epidermis. Melanocytes are pigment cells that help the skin tan.

Melanocytes produce melanin – a protein pigment which performs the very specific body function of protecting skin from overexposure to ultraviolet light. Thus, the presence of melanin in the skin colors it and protects it.

Everyone has roughly the same number of melanocytes in the body—about five million. Your body’s melanocytes naturally will produce a certain amount of melanin based on your heredity, which is why people have different skin colors. For example, the skin of African-Americans contains more melanin, creating a black or brown color, while the skin of Caucasians has less melanin and is pale.

UVA, UVB and the Tanning Process
Melanocytes are prompted to produce additional melanin whenever ultraviolet light waves touch them, thereby making the skin darker to protect the body from additional exposure. This produces a tan—literally, a browning of the skin. The color of the tan ultimately depends on heredity and previous exposure to ultraviolet light, two factors which predetermine the amount of melanin your skin will contain. This explains why some fair-skinned people can get dark tans and others cannot.

Of course, ultraviolet light can affect the skin in other ways. In excessive doses, it can cause sunburn – a reddening caused by the swelling or bursting of tiny blood vessels in the skin. Repeated burning is believed to be the greatest risk factor for long-term skin damage, which is why it is so important to prevent sunburn.

UVA and UVB waves have specific roles in the tanning process which are determined by their effects on skin. Although all ultraviolet light is capable of tanning skin, UVA is more efficient at certain functions in the tanning process and UVB is more efficient at certain parts of tanning. For instance, melanin produced when your skin is exposed to UV light is naturally pinkish in tone. But ultraviolet light also oxidizes the melanin, turning it brown.

UVB is more efficient at signaling melanocytes in your skin to begin producing more melanin.
UVA is more efficient at oxidizing the melanin your skin has already produced, turning it brown.

What Is Skin Damage?
You need to understand that technically, on the micro-level, any ultraviolet light exposure causes “skin damage.” But you also need to know that, on the macro-level, UV exposure is natural and necessary to lead a healthy life and simply calling UV exposure “damage” to your skin is more misleading than it is true.

What is sometimes called “damage” to the skin from non-burning UV exposure is actually just the skin’s way of protecting itself from sunburn. If your body can develop a tan, doing so is natural. It is what your body is designed to do. It is one of the ways your body protects itself.

Saying that UV light damages the skin, and therefore you should avoid UV light, is like saying that water causes drowning, and therefore you should avoid water. Just like water, we need UV light to live. So calling UV exposure “damage” is an oversimplification that misrepresents what your body as a whole is designed to do. The sum of research conducted to date indicates that repeated overexposure and sunburning are the primary sun-related factors responsible for an increased risk of permanent skin damage. That’s why the prevention of sunburn and overexposure are so important.